What GPS HDOP and PDOP do you get? A good GPS fix will have an HDOP of <1.5, mine is mostly between 1.2 and 1.8
An HDPO of over 5 is a pretty poor position.
HDOP = horizontal dilution of position, PDOP = positional dilution of precision. The lower the HDOP, the more accurate the position calculation from the GPS.

For the Mk4, the left-front speed sensor is connected to pin 10 on the nav. Don't blindly play with the loom, you should confirm that it is wired correctly as per the instructions.